This invention relates to timer circuits and power control circuits for electrical appliances and more particularly to control circuitry which is responsive to operator touch.
In microwave cooking appliances, the nature of the heating phenomenon is that of stressing certain of the molecules of the products to be heated by using an electromagnetic field, commonly in the heating frequency range of 2450 MHz. Such heating causes rapid cooking of food products and cooking time may typically be measured in seconds rather than hours and minutes as with more conventional cooking devices. It is important in such a device, therefore, to provide sufficiently accurate time control of the cooking cycle to insure that food is neither over-cooked nor undercooked. The environment in which microwave ovens are normally placed, however, such as a restaurant kitchen, has a deleterious effect upon control circuit operation. Moisture, food deposits, and grease build up on oven controls may cause a malfunction and presents a difficult cleaning problem.
One approach taken to solution of this problem has been the use of tough sensitive switches as controls. This allows for the control panel to be constructed as a smooth, easily cleaned plate having no moving parts. An example of such a control used with an oven-range combination is disclosed in U.S. Pat. No. 3,819,906. The touch sensitive circuit there disclosed is comprised of a plurality of level detectors, each detector connected to receive an oscillating signal from an associated "proximity switch" which includes a touch plate. When one of the touch plates is touched, a capacitor divider is formed which attenuates the signal being applied by the proximity switch to its associated detector. The detector senses the attenuation and switches its output signal.
Another principle of operation used in prior art touch sensitive circuits is disclosed in U.S. Pat. No. 3,492,440. That patent discloses a circuit in which the capacitance to ground of an operator completes a circuit and allows for an oscillator output to be applied to a level detector.
Touch sensitive circuitry used in the past has generally been susceptible to false actuations. Where the capacitance to ground of an operator is used to attenuate a signal or complete a circuit, there is a possibility of false actuation since a resistance to ground can also cause circuit actuation. The environment in which commercial microwave ovens operate is such that the possibility continually exists for grease and other foreign matter to form a resistance to ground from the various touch plates and thus cause the controls to malfunction.